omap2+: Remove useless Makefile line

Commit f41caddb ("omap2+: Use Kconfig
symbol in Makefile instead of obj-y") reorganized this Makefile. But,
for some reason, it also added references to CONFIG_MACH_ENCORE and
board-omap3encore.o. But there's no Kconfig symbol MACH_ENCORE and
there's no file board-omap3encore.c. This line can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tony Lindgren <tony@atomide.com>